Photovoltaic cells based on nanoscale structures

ABSTRACT

Novel structures of photovoltaic cells (also treated as solar cells) are provided. The Cells are based on the nanometer-scaled wire, tubes, and/or rods, which are made of the electronics materials covering semiconductors, insulator or metallic in structure. These photovoltaic cells have large power generation capability per unit physical area over the conventional cells. These cells can have also high radiation tolerant capability. These cells will have enormous applications such as in space, in commercial, residential and industrial applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/522,134 filed on Aug. 19, 2004.

FIELD OF INVENTIONS

This patent specification relates to structures of photovoltaic cells(also solar cells). More specifically, it relates to structures ofphotovoltaic cells comprising numerous nanometer-scale wires, rodsand/or tubes to have large power generation capability per unit area.The photovoltaic cells have also highly radiant tolerant, necessary forspace applications. These photovoltaic cells can be used in commercial,residential, and also industrial application for power generation.

BACKGROUND OF THE INVENTIONS

Photovoltaic cells where light is converted into electric power to befed to external loads electrically connected to the photovoltaic cellshave been prevailing in a wide range of application fields such asconsumer electronics, industrial electronics and space exploration. Inconsumer electronics, photovoltaic cells that consist of materials suchas amorphous silicon are choices for a variety of inexpensive and lowpower applications. Typical conversion efficiency, i.e. the solar cellconversion efficiency, of amorphous silicon based photovoltaic cellsranges between ˜10% [Yamamoto K, Yoshimi M, Suzuki T, Tawada Y, OkamotoT, Nakajima A. Thin film poly-Si solar cell on glass substratefabricated at low temperature. Presented at MRS Spring Meeting, SanFrancisco, April 1998.]. Although the fabrication processes of amorphoussilicon based photovoltaic cells are rather simple and inexpensive, onenotable downside of this type of cell is its vulnerability todefect-induced degradation that decreases its conversion efficiency.

In contrast, for more demanding applications such as industrial solarpower generation systems, either poly-crystalline or single-crystallinesilicon is the choice because of more stringent requirements for betterreliability and higher efficiency than the applications in consumerelectronics. Photovoltaic cells consisting of poly-crystalline andsingle-crystalline silicon generally offer the conversion efficiencyranging ˜20% and ˜25% [Zhao J, Wang A, Green M, Ferrazza F Novel 19.8%efficient ‘honeycomb’ textured multicrystalline and 24.4%monocrystalline silicon solar cell. Applied Physics Letters 1998; 73:1997-1993.] respectively. As many concerns associated with a steepincrease in the amount of the worldwide energy consumption are raised,further development in industrial solar power generation systems hasbeen recognized as a main focus.

Group II-VI compound semiconductors, for example CdTe and CdS, have beeninvestigated in the context of having industrial solar power generationsystems manufactured at a lower cost with maintaining a moderateconversion efficiency, resulted in a comparable conversion efficiency˜17% [Wu X, Keane J C, Dhere R G, DeHart C, Duda A, Gessert T A, AsherS, Levi D H, Sheldon P. 16_(—)5%-efficient CdS/CdTe polycrystallinethin-film solar cell. Proceedings of the 17th European PhotovoltaicSolar Energy Conference, Munich, 22-26 Oct. 2001; 995-1000.] to thosefor the single crystalline silicon photovoltaic devises, however toxicnatures of these materials are of great concerns for environment.

Group I-III-VI compound semiconductors, such as CuInGaSe₂, have beenalso extensively investigated for industrial solar power generationsystems. This material can be synthesized potentially at a much lowercost than its counterpart, single crystalline silicon, howeverconversion efficiency, ˜19%, comparable to that of single crystallinesilicon based cells can be obtained, so far, by only combining with thegroup II-VI compound semiconductor cells [Contreras M A, Egaas B,Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, Noufi R. Progresstoward 20% efficiency in Cu(In, Ga)Se polycrystalline thin-film solarcell. Progress in Photovoltaics: Research and Applications 1999, 7:311-316.], which again raise issues associated with toxic natures ofthese materials.

A type of photovoltaic cells designed for several exclusive applicationswhere the main focus is high conversion efficiency generally consists ofgroup III-V semiconductors including GaInP and GaAs. Synthesis processesof single crystalline group III-V are in general very costly because ofsubstantial complications involved in epitaxial growth of group III-Vsingle crystalline compound semiconductors. Typical conversionefficiency of group III-V compound semiconductor based photovoltaiccells, as these types of photovoltaic cells are intended to be, can beas high as ˜34% when combined with germanium substrates, another veryexpensive material [King R R, Fetzer C M, Colter P C, Edmondson K M, LawD C, Stavrides A P, Yoon H, Kinsey G S, Cotal H L, Ermer J H, Sherif RA, Karam N H. Lattice-matched and metamorphic GaInP/GaInAs/Geconcentrator solar cells. Proceedings of the World Conference onPhotovoltaic Energy Conversion (WCPEC-3), Osaka, May 2003, to bepublished.].

All types of photovoltaic cells in the prior arts described above, nomatter what materials a cell is made of, essentially falls into onespecific type of structure as in FIG. 1. Shown in FIG. 1 is aphotovoltaic cell comprising a thick p-type semiconductor layer 101 anda thin n-type semiconductor layer 102 formed on an electricallyconductive substrate 100. A pn-junction 103 is formed at the interfacebetween the p-type semiconductor layer 101 and the n-type semiconductorlayer 102. Incident light 104 entering the cell generate electron-holepairs after being absorbed by the p and also n-type semiconductor layers101 and 102. The incident light generates electrons 105 e and also holes105 h in the region near the pn-junction 103 and also 106 e and 106 h inthe region far from the pn-junction 103. The photo generated electrons(and holes) 105 e and 106 e (hereafter considering only electronics,i.e. minority carriers in p-type semiconductors, and the sameexplanation is applicable for holes, minority carriers in n-typesemiconductors, also) diffusing toward the pn-junction 103 and enteringthe pn-junction 103 contribute to photovoltaic effect. This is also viceversa for the holes, existing as minority carriers in n-typesemiconductor 102. The two key factors that substantially impact theconversion efficiency of this type of photovoltaic cell are photocarrier generation efficiency (PCGE) and photo carrier collectionefficiency (PCCE).

The PCGE is the percentage of the number of photons entering a cell andcontributing to the generation of photo carriers, which needs to be,ideally, as close as 100%. On the other hand, the PCCE is the percentageof the number of photo-generated electrons 105 e and 106 e reaching thepn-junction 103 and contributing to the generation of photocurrent. Fora monochromatic light, the PCGE of ˜100% can be achieved by simplymaking the p-type layer 101 thicker, however, electrons 106 e generatedat the region far away from the pn-junction 103 cannot be collectedefficiently due to many adverse recombination processes that preventphoto generated carriers from diffusing into the pn-junction 103, thusthe basic structure of current photovoltaic cells has its own limitationon increasing the conversion efficiency. Both PCGE and PCCE are mainlydependent on material and structure of the photovoltaic cells, andtoday's photovoltaic cells are structured in such a way that (a) wideranges of solar spectrum cannot be absorbed due to its materiallimitation, and (b) photo carrier's collection efficiency is lower dueto its inherent structure. Besides, today's solar cell material is nothighly radiation-tolerant. In space application specially, photovoltaiccells should have a structure and material systems, which could generatehigh-power per unit area and also to highly radiation tolerant.

For both commercial and space applications, therefore, it would bedesirable to have photovoltaic cell structures where both the PCGE andthe PCCE can be increased simultaneously by having a photo absorptionregion that is thick enough to capture all the photons entering the celland a pn-junction that is located at as close to the photo absorptionregion as possible. It would be further desirable to have, withmaintaining ideal PCGE and PCCE, different materials having photoresponses at different spectrum to efficiently cover a wide range ofspectrum of light that enters a photovoltaic cell. It would be furtherdesirable to have a large junction area within a given volume of aphotovoltaic cell so that generated electric power that is proportionalto the junction area can be maximized.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide the structuresof the photovoltaic cells, which could have the high power generationcapability per unit area over conventional counterpart, mentioned as theprior arts.

According to this invention, the photovoltaic cell can be made to highlyradiation tolerant.

Structures of photovoltaic cells comprising single or plurality ofnanometer(s)-scale wires, rods or tubes consisting of various electronicmaterials are described. The surfaces of the nanometer(s)-scale wires,rods or tubes formed on a supporting substrate are connected to anotherelectronic material or several different electronic materials, forming alarge area of pn- or Schottky junctions on the surfaces of thenanometer(s)-scale wires, rods or tubes. The created large area pn- orSchottky junctions on the surface of the plurality of nanometer(s)-scalewires, rods or tubes form built-in potential by which photo generatedelectrons and holes are swept away, leading to photovoltaic effect.

According to this invention, the nanometers-scaled wires, rods or tubesare made of electronic materials such as semiconductors, insulator ormetals, or their combination, and they are formed on base substrateswith geometries in which the axial direction of nanometers-scaled wires,rods or tubes are either in perpendicular or parallel with respect tothe surface normal of the base substrates. According to this invention,the nanometer-scaled wires, rods or tubes could be made of any types,elementary or compound, of semiconductors such as Si, Ge, C, ZnO, BN,Al₂O₃, AlN, Si:Ge, CuInSe, II-VI and III-V, etc., or their combinations.The nanometer(s)-scaled tube could be made of semiconductor, insulator,or metallic type tubes such as carbon nano-tubes.

It is also another object of this invention to provide the structures ofthe photovoltaic cells based on the carbon-nanotubes or semiconductorwires or rods which could provide more junction area per unit physicalarea, which results in increasing the power generation per unit areaover conventional photovoltaic cells.

This invention offers to generate power 100 times per unit area adbeyond over conventional photovoltaic cells. Also, the proposedphotovoltaic cells are highly radiation tolerant, necessary in the spaceapplication. The main advantages of these inventions are that today'shighly matured semiconductor process technologies can be used tofabricate the photovoltaic cell which has the power generationcapability a few order and beyond as compared with that of conventionalphotovoltaic cell.

Other objects, features, and advantages of the present invention will beapparent from the accompanying drawings and from the detaileddescription that follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in conjunction with theappended drawings wherein:

FIG. 1 is the schematic showing the cross-sectional view of aconventional photovoltaic cell structure. This is the explanatorydiagram showing the prior-art of today's photovoltaic cell.

FIG. 2 is the schematic showing the cross-sectional view of aphotovoltaic cell structure consisting of the nanometer(s)-scale wires,or rods, vertically arranged, in the first embodiment in accordance tothe present invention.

FIG. 3 is the schematic showing the cross-sectional view of aphotovoltaic cell structure consisting of the nanometer(s)-scale tubesvertically arranged, in the second embodiment in accordance to thepresent invention.

FIG. 4 is the schematic showing the cross-sectional view of aphotovoltaic cell structure consisting of the nanometer(s)-scale wires,rods or tubes and multi-layered semiconductors, bandgaps of which arerelates to the different spectrum of the solar spectrum, in the thirdembodiment in accordance to the present invention.

FIG. 5 is the schematic showing the cross-sectional view of aphotovoltaic cell structure consisting of the nanometer(s)-scale wires,rods or tubes, formed on pyramid or triangular shaped surface to achievelarge junction area, in the fourth embodiment in accordance to thepresent invention.

FIGS. 6A and 6B are the schematic showing the side-view andcross-sectional views of a photovoltaic cell structure consisting of thenanometer(s)-scale wires, rods or tubes, arranged horizontally, parallelto the substrate to achieve large junction area, in the fifth embodimentin accordance to the present invention.

FIG. 7 is the schematic showing the cross-sectional view of aphotovoltaic cell structure consisting of the nanometer(s)-scale wires,rods or tubes formed on the semiconductor/insulator self-assembled dotsor islands, arranged vertically to the substrate to achieve largejunction area, in the sixth embodiment in accordance to the presentinvention.

FIG. 8 is the schematic showing the side-view of a photovoltaic cellstructure consisting of the nanometer(s)-scale wires, rods or tubes, inthe seventh embodiment in accordance to the present invention.

DETAILED DESCRIPTION

According to a preferred embodiment illustrated in FIG. 2, shown is aphotovoltaic cell comprising plurality of nanometer(s)-scale wires orrods 201 electrically connected to an electrode 200. Thenanometer(s)-scale wires or rods 201 can have metallic electricalconduction, p-type or n-type semiconductor electrical conduction. Thenanometer(s)-scale wires or rods are further surrounded by an electronicmaterial 202 having metallic electrical conduction, p-type or n-typesemiconductor electrical conduction. The electronic material 202 isfurther electrically connected to an electrode 203. The electrode 200has direct electrical contact to neither the electrical material 202 northe electrode 203. As described above, the electrode 200 is intended toserve as a common electrode that connects all wires or rods 201. Theelectrode 203 is provided for the electronic material 202. The interfacebetween the nanometer scale wires or rods 201 and the electronicmaterial 202 form pn- or Schottky junctions where built-in potential forboth electrons and holes is generated.

According to this invention, alternatively the nanometer(s)-scale wiresor rods 201 can be formed on separate substrate (not shown here), andthe electrode 203 can be formed on the substrate to have common contactfor each nanometer(s)-scale rods or tubes 201, necessary for creatingjunction. In way of an example not way of limitation, thenanometer(s)-scale wires or rods 201 can be made of n-type semiconductorand the electric material 202 that surrounds the nanometer(s)-scalewires or rods 201 can be made of p-type semiconductor. Incident light204 enters the photovoltaic cell through either the electrode 203 or theelectrode 200 (In FIG. 2, the incident light enters the photovoltaiccell through the electrode 200). As the incident light 204 travelsthrough the electronic material 202, a numerous number of electrons 205in the region near the electrode 200 and electrons 206 in the region farfrom the electrode 200 are generated. It should be pointed out thatelectrons are apparently generated all over the region along thethickness of the electric material 202. In addition, as the incidentlight 204 travels through the nanometer(s)-scale wires or rods 201, anumerous number of holes 207 in the region near the electrode 200 andholes 208 in the region far from the electrode 200 are generated. Italso should be pointed out that holes are apparently generated all overthe region along the thickness of the nanometer(s)-scale wires or rods201. Photo-generated electrons 205 and 206 in the electronic material202 made of p-type semiconductor and photo-generated holes 207 and 208in the nanometer(s)-scale wires or rods 201 made n-type semiconductor,then diffuse toward pn-junctions, created at the interface between thenanometer(s)-scale wires rods 201 and the electronic material 202. Atthe pn-junctions, the electrons and the holes are swept away by built-inpotential, thus photovoltaic effects set in.

Apparent advantage of this invention over conventional photovoltaiccells is directly associated with the fact that, unlike conventionalphotovoltaic cells, pn-junctions are almost parallel to the direction towhich incident light 204 travels, i.e., for all photo generated carriersin the electronic material 202, no matter where they are generated, thedistance the photo generated carriers have to diffuse to reach thepn-junctions is within the range of the distance between twonanometer(s)-scale wires or rods existing next to each other andindependent to the location where they are generated. Furthermore, forall photo generated carriers in the nanometer(s)-scale wires or rods201, no matter where they are generated, the distance the photogenerated carriers have to diffuse to reach pn-junctions is within therange of the diameter of the nanometer(s)-scale rods 201. On the otherhand, as explained in the description for the prior art shown in FIG. 1,in conventional photovoltaic cells where pn-junctions are perpendicularto the direction to which incident light travels, the photo generatedcarriers generated in region far away from pn-junctions need to diffusemuch longer distance (diffusion-length) than that for the photogenerated carriers generated near the pn-junctions, thus they have agreater chance to recombine without contributing to photovoltaiceffects. Therefore in this invention, PCCE is expected to be much higherthan that in conventional photovoltaic cells. In addition, it is evidentthat the total effective area that contributes to photovoltaic effect inthis invention can be increased by 25 times and beyond for a given cellsize with realistic assumptions on the dimension of thenanometer(s)-scale structures in the cell.

In an alternative preferred embodiment shown in FIG. 3, a photovoltaiccell comprises plurality of nanometer(s)-scale tubes 301 areelectrically connected to an electrode 300. The nanometer(s)-scale tubes301 can have metallic electrical conduction, p-type or n-typesemiconductor electrical conduction. The nanometer(s)-scale tubes arefurther surrounded by an electronic material 302 having metallicelectrical conduction, p-type or n-type semiconductor electricalconduction. The inside of the nanometer(s)-scale tubes 301 can be eitherempty or filled up with an electronic material 303 having metallicelectrical conduction, p-type or n-type semiconductor electricalconduction. Two electronic materials 302 and 303 are furtherelectrically connected to an electrode 304. The electrode 304 has directelectrical contact to neither the electrode 300 nor thenanometer(s)-scale tubes 301. The electrode 304 is intended to serve asa common electrode that connects all materials inside of the tubes 303and outside of the tubes 302. The interface between thenanometer(s)-scale tubes 301 and the electronic materials 302/303 formpn- or Schottky junctions, thus there are pn- or Schottky junctions onboth sides, inside and outside, of the nanometer(s)-scale tubes 301.

According to this invention, alternatively the nanometer(s)-scale tubes301 can be formed on the substrate (not shown here), and the electrode304 can be made on the substrate to have a common contact for eachnanometer(s)-scale rods or tubes 301, necessary for creating junction.

In way of an example not way of limitation, the nanometer(s)-scale tubes301 can be made of metal and the electronic material 302 that surroundsthe nanometer(s)-scale tubes 301 and the electronic material 303 thatfills up the inside of the nanometer(s)-scale tubes 301 can be made ofp-type semiconductor, thus a sandwich structure 302/301/303 forms twoSchottky junctions on both sides of the metallic nanometer(s)-scaletubes 301. Incident light 305 enters the photovoltaic cell througheither the electrode 304 or the electrode 300 (In FIG. 3, the incidentlight enters the photovoltaic cell through the electrode 300). As theincident light 305 travels through the electronic material 302 and 303,numerous numbers of electrons 306 and 307 (of electron-hole pairs) aregenerated. It should be pointed out that electrons (of electron-holepairs) are apparently generated all over the region along the thicknessof the nanometer(s)-scale tubes 301. Photo-generated electrons in theelectronic material 302 and the electric materials 303 made of p-typesemiconductor, then diffuse toward Schottky junctions in the sandwichstructure 302/301/303. At the Schottky junctions, the diffused electronsare swept away by built-in potential, thus photovoltaic effects set in.

In addition to the common advantages already described for thephotovoltaic cell in FIG. 2, since, in this invention, both inside andoutside of the nanometer(s)-scale tubes 301 form junctions, theeffective area that contributes to photovoltaic effects are roughlydouble the area provided by the cell in FIG. 2, thus the total electricpower can be increased 50 times and beyond for a given cell sizecompared to conventional photovoltaic cells.

In an alternative preferred embodiment illustrated in FIG. 4, aphotovoltaic cell comprises plurality of nanometer(s)-scale wires orrods 401 are electrically connected to an electrode 400. It is obviousfor a person of ordinary skill in the art to recognize the followingdescription applies to a photovoltaic cell comprises plurality ofnanometer(s)-scale tubes, instead of wires or rods, as in FIG. 3. Thenanometer(s)-scale wires or rods 401 can have metallic electricalconduction, p-type or n-type semiconductor electrical conduction. Thenanometer(s)-scale wires or rods are further surrounded by multiplelayers of different electronic materials 402˜404 having metallicelectrical conduction, p-type or n-type semiconductor electricalconduction. The number of layers shown in FIG. 4 is not the limitation,and it is apparent that the number of layers can be a wide range ofnumbers depending primarily on the thickness of each layer and theheight of the nanometer(s)-scale wires or rods 401. The multiple layerscan be dissimilar semiconductors having different bandgaps appropriatelytuned to cover a wide range of spectrum of the light entering the cellas described later. The electronic material 404 is further electricallyconnected to an electrode 405. The electrode 400 has direct electricalcontact to neither the electrical material 402 nor the electrode 405.The electrode 400 is intended to serve as a common electrode thatconnects all wires or rods 401. The electrode 405 is provided for theelectronic material 404. The interface between the nanometer(s)-scalewires or rods 401 and the electronic material 402˜404 form pn- orSchottky junctions where built-in potential for both electrons and holesis generated.

In way of an example not way of limitation, the nanometer(s)-scale wiresor rods 401 can be made of n-type semiconductor and the electronicmaterial 402˜404 that surrounds the nanometer(s)-scale wires or rods 401can be made of three different types of p-type semiconductors havingdifferent bandgaps. Incident light 406 that contains a broad-spectrumrange enters the photovoltaic cell through either the electrode 405 orthe electrode 400 (In FIG. 4, the incident light enters the photovoltaiccell through the electrode 400). As the incident light 406 travelsthrough the electronic material 402˜404, a specific spectrum range inthe incident light 406 is absorbed in a specific layer in the multiplelayers of electronic materials 402˜404, in that, short, middle and longwavelengths in the incident light 406 can be absorbed subsequently inthe layers 402, 403 and 404 respectively, then numerous number ofelectron 407 (and holes) are generated in each layers. Photo-generatedelectrons in the electronic material 402˜404 made of p-typesemiconductor (and vice versa for the holes (not shown here)), thendiffuse toward the pn-junctions, created at the interface between thenanometer(s)-scale wires or rods 401 and the multi layered electronicmaterial 402˜404. At the pn-junctions, the electrons 407 are swept awayby built-in potential, thus photovoltaic effects set in. Apparently, inaddition to the common advantages already described in FIG. 2˜FIG. 3over conventional cells in FIG. 1, the additional advantage of the cellin FIG. 4 over the photovoltaic cells described in FIG. 2˜FIG. 3 is tohave a capability of covering a wide range of spectrum contained inincident light and converting a wide range of spectrum to photogenerated carriers. Dozens of different layers could be stacked to catchphotons at all energies, to make absorb wide band of solar spectrum,from lower wavelengths (as low as X-ray) to longer wavelength (e.g. longinfrared). Addition of the multiple junction of different materialswhich could absorb wide solar spectrum plus the increasing of thejunction area with using of the rod, wires, or tubes help to increasethe electrical power energy 50 times and beyond as compared with theconventional solar cell of same size. According to this invention,dozens of materials, which could absorb wide solar spectrum, may or maynot require the lattice mismatch with the rod, wires, or tubes. Latticematched material could increase further increase of the power generationdue to reduction of the recombination.

According to this invention, the rods, or wires could be GaN materials(n or p type) and the dozens of the materials could be In_(1-x)Ga_(x)N(p or n type, opposite to GaN rods). With increasing of the Ga contents,the band-gap of InGaN can be increased to close to ˜3.4 eV which is sameas that of the GaN. With increasing of the In contents in InGaN, theband gap can be reduced to ˜0.65 eV. Photons with less energy than theband gap slip right through. For example, red light photons are notabsorbed by high-band-gap semiconductors. While photons with energyhigher than the band gap are absorbed—for example, blue light photons ina low-band gap semiconductor—their excess energy is wasted as heat.

According to this invention, alternatively the rods, or wires could beIII-V based materials (n or p type) for example InP and the dozens ofthe materials could be III-V based material for example In_(1-x)Ga_(x)As(p or n type, opposite to InP rods). In this case, with adjusting of Incontents, band gap can be tuned and thereby the wide spectrum of thesolar energy can be absorbed.

According to this invention, alternatively the rods, or wires could beII-V based materials (n or p type) for example CdTe and the dozens ofthe materials could be II-VI based material for example CdZnS (p or ntype, opposite to CdTe rods). In this case, with adjusting of Zncontents, band gap can be tuned and thereby the wide spectrum of thesolar energy can be absorbed.

According to this invention, alternatively the rods, or wires could beSi (or amorphous Silicon materials (n or p type) and the dozens of thematerials could be Si: Ge alloy (p or n type, opposite to Si rods). Inthis case, with adjusting of Ge contents, band gap can be tuned andthereby the wide spectrum of the solar energy can be absorbed.

According to this invention, alternatively the rods, or wires could beSi, InP, or CdTe (n or p type) and the dozens of the materials, could bedifferent material which could make the junction with the rods (wires ortubes) and each type of material has the specific band gap for absorbingthe specific range of solar spectrum. In this way also wide range ofsolar spectrum can be absorbed, and with increasing of the junction area(due to use of the rods, wires, or tubes), the electrical powergeneration could be increased tremendously 50 times and beyond.

According to this invention, alternatively the nanometer(s)-scale rods401 can be formed on the substrate (not shown here), and the electrode405 can be made on the substrate to have a common contact for eachnanometer(s)-scale rods or tubes 401, necessary for creating junction.

According to this invention, alternatively the nanometer(s)-scale tubes(not shown here) can be formed instead of nanometer(s)-scale rods 401and furthermore those nanometer(s)-scale tubes can be either empty orfilled up with an electronic material having metallic electricalconduction, p-type or n-type semiconductor electrical conduction.

In another preferred embodiment shown in FIG. 5, plurality ofphotovoltaic cells comprising plurality of nanometer(s)-scale wires orrods 501 are randomly and electrically connected to an electrode thathas arbitrary shapes 500 (In FIG. 5, triangular shape is illustrated,however any arbitrary shapes can be applicable here) further connectedan electrode 502. It is obvious for a person of ordinary skill in theart to recognize the following description applies to a photovoltaiccell comprises plurality of nanometer(s)-scale tubes, instead of wiresor rods, as in FIG. 3. The nanometer(s)-scale wires or rods 501 can havemetallic electrical conduction, p-type or n-type semiconductorelectrical conduction. The nanometer(s)-scale wires or rods are furthersurrounded by an electronic material 503 having metallic electricalconduction, p-type or n-type semiconductor electrical conduction. Theelectronic material 503 is further electrically connected to anelectrode 504. The electrodes 500 and 502 have direct electrical contactto neither the electrical material 503 nor the electrode 504. Theinterface between the nanometer(s)-scale wires or rods 501 and theelectronic material 502 form pn- or Schottky junctions where built-inpotential for both electrons and holes is generated. The way thisphotovoltaic cell operates is just the same way in other photovoltaiccells illustrated in FIG. 2˜FIG. 4, therefore, the uniquecharacteristics for this photovoltaic cell in FIG. 5 is the fact thatthe nanometer(s)-scale wires, rods or tubes, are connected to electrodesthat can have, instead of planar surface, three-dimensionally arbitraryshape. This structure helps to increase the effective junction area andPCCE is expected to be much higher than in photovoltaic cells describedin FIGS. 2 to 4.

FIGS. 6A and 6B, another preferred embodiment, are schematics showingthe side and cross-sectional view of a photovoltaic cell structureconsisting of nanometer(s)-scale wires, rods or tubes, arrangedhorizontally, parallel to a substrate to achieve large junction areawith respect to the incident light entering the cell perpendicular tothe substrate surface. As in FIG. 6A, a substrate 600 that has metallic,or n-type or p-type semiconductor electrical conduction, has pluralityof vertical walls 601 made of a material that has metallic, or n-type orp-type semiconductor electrical conduction. Electrodes 606 are providedto the substrate 600 for electrical connection to the photovoltaic cell.Metal or semiconductor nanometer(s)-scale wires or rods 602 are formedperpendicular to the vertical walls 601, bridging two adjacent verticalwalls 601. It should be noted that the metallic or semiconductor wires,rods or tubes 602 could also be tubes rather than wires or rods. Thenanometer(s)-scale rods 602 can be made of either a similar or adissimilar material to that of the vertical walls 601. Thenanometer(s)-scale wires or rods 602 are further surrounded by anelectronic material 603 made of a semiconductor material. The electronicmaterial 603 is electrically connected to an electrode 604, however theelectrode 604 is electrically isolated from the substrate 600 by aninsulator 605 and from the vertical walls 601.

In way of an example not way of limitation, in FIG. 6B, thenanometer(s)-scale rods 602 can be made of n-type semiconductor and theelectronic material 603 that surrounds the nanometer(s)-scale wires orrods 602 can be made of p-type semiconductors, thus forming large areapn-junction 607 at the interface between the nanometer(s)-scale wires orrods 602 and the electronic material 603. As the incident light 608enters the cell, carriers are generated both in the nanometer(s)-scalerods 602 and the electronic material 603, diffusing into the pn-junction607 and contributing to photovoltaic effects.

According to this invention, the surrounding material 603 can be singleor plurality layers of materials having different band gaps, whichcorrespond to the different absorption wavelength, contained in theincident light, as described in FIG. 4.

FIG. 7, another preferred embodiment, is a schematic showing thecross-sectional view of a photovoltaic cell structure consisting ofplurality of nanometer(s)-scale wires, rods or tubes 701 formed onsemiconductor/insulator self-assembled dots or islands 702, arrangedvertically to a substrate 700. The nanometer(s)-scale wires, rods ortubes 701 can be The nanometer(s)-scale wires, rods or tubes can be madeof a variety of electronic materials such as metals and semiconductors(Elemental semiconductors such as Si, Ge and C and compoundsemiconductors). 701 can also be directly formed on the electronicmaterial substrate 700 (not shown here). In the case ofnanometer(s)-scale tubes, the tubes can be empty or filled up by theelectronic materials (not shown here), as described in FIG. 3. Thenanometer(s)-scale wires, rods or tubes 701 are further connected to anelectrode 704. The nanometer(s)-scale wires, rods or tubes 701 are alsosurrounded by a semiconductor 703 for which electrode 705 are provided.

In way of an example not way of limitation, in FIG. 7, thenanometer(s)-scale wires, rods or tubes 701 can be made of n-typesemiconductor and the electronic material 703 that surrounds thenanometer(s)-scale wires, rods or tubes 701 can be made of p-typesemiconductors, thus forming large area pn-junction 706 at the interfacebetween the nanometer(s)-scale rods 701 and the electronic material 703.As the incident light 707 and 708 enter the cell, carriers are generatedboth in the electronic material 703 and the nanometer(s)-scale wires,rods or tubes 701, diffusing into the pn-junction 706 and contributingto photovoltaic effects.

In another preferred embodiment, FIG. 8 illustrates a side view of aphotovoltaic cell comprising of nanometer(s)-scale wires, rods or tubes,arranged horizontally, parallel to a substrate to achieve large junctionarea with respect to the incident light entering the cell along thesurface normal of the substrate 800. As in FIG. 8, a substrate 800 thathas metallic, or n-type or p-type semiconductor electrical conduction,is covered with an electrical insulation layer 801. Pluralities ofnanometer(s)-scale tubes 802 are formed horizontally on the insulatinglayer 801. The nanometer(s)-scale tubes 802 are further surrounded by anelectronic material 803 that has metallic, or n-type or p-typesemiconductor electrical conduction. An electrode 804 is provided on theelectronic material 803. The both ends of the plurality ofnanometer(s)-scale tubes 802 are connected to electrodes 805, thus theplurality of nanometer(s)-scale tubes 802 are electrically accessiblefrom the electrodes 805, and the interface between the plurality ofnanometer(s)-scale tubes 802 and the electronic material 803 formseither pn-junction of Schottky junctions.

In way of an example not way of limitation, the nanometer(s)-scale tubesmade of carbon atoms 802 that have metallic electrical conduction can besurrounded by the electronic material 803 made of p-type semiconductors,thus forming Schottky-junction at the interface between thenanometer(s)-scale tubes 802 and the electronic material 803. As theincident light 807 enters the cell, electrons are generated in theelectronic material 803, and then the photo-generated electronsdiffusing into the Schottky-junction contribute to photovoltaic effects.

According to this invention, the nanometer(s)-scale wires, rods ortubes, mentioned in the preferred embodiments, can be any kinds ofelectronics materials covering semiconductor, insulator or metal.

According to this invention, the nanometer sized rods, wire or tubes canbe made from the semiconductors such as Si, Ge, or compoundsemiconductors from III-V or II-VI groups. As an example for rods, wire,or tubes, InP, GaAs, or GaN III-V compound semiconductor can be used andthey can be made using standard growth process for example, MOCVD, MBE,or standard epitaxial growth. According to this invention, theself-assembled process can also be used to make wires, rods, or tubesand their related pn-junction to increase the junction area. These rods,wire, or tubes can be grown on the semiconductors (under same group orothers), polymers, or insulator. Alternatively, according to thisinvention, these rods, wire, or tubes, can be transferred to the foreignsubstrate or to the layer of foreign material. The foreign substrate orthe layer of material can be any semiconductor such as Si, Ge, InP,GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substrate can coveralso all kinds of polymers or ceramics such as AlN, Silicon-oxide etc.

According to this invention, the nanometer sized rods, wire or tubesbased on II-VI compound semiconductor can also be used. As an exampleCdTe, CdS, Cdse, ZnS, or ZnSe can also be used, and they can be madeusing standard growth process for example, sputtering, evaporation,MOCVD, MBE, or standard epitaxial growth. According to this invention,the self-assembled process can also be used to make wire, rods, or tubesand their related pn-junction to increase the junction area. These rods,wire, or tubes can be grown on the semiconductors (under same group orothers), polymers, or insulator. Alternatively, according to thisinvention, these rods, wire, or tubes, can be transferred to the foreignsubstrate or to the layer of foreign material. The foreign substrate orthe layer of material can be any semiconductor such as Si, Ge, InP,GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substrate can coveralso all kinds of polymers or ceramics such as AlN, Silicon-oxide etc.

According to this invention, the nanometer sized rods, wire or tubes canbe made from the carbon type materials (semiconductor, insulators, ormetal like performances) such as carbon nano-tubes, which could besingle, or multiple layered. They can be made using standard growthprocess for example, MOCVD, MBE, or standard epitaxial growth. Accordingto this invention, the self-assembled process can also be used to makewires, rods, or tubes and their related pn-junction to increase thejunction area. These tubes can be grown on the semiconductors (undersame group or others), polymers, or insulator. Alternatively, accordingto this invention, these rods, wire, or tubes, can be transferred to theforeign substrate or to the layer of foreign material. The foreignsubstrate or the layer of material can be any semiconductor such as Si,Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe, HgCdTe, etc. The substratecan cover also all kinds of polymers or ceramics such as AlN,Silicon-oxide etc.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that theparticular embodiments shown and described by way of illustration are inno way intended to be considered limiting. Therefore, reference to thedetails of the preferred embodiments is not intended to limit theirscope. Although the invention has been described with respect tospecific embodiment for complete and clear disclosure, the appendedclaims are not to be thus limited but are to be construed as embodyingall modification and alternative constructions that may be occurred toone skilled in the art which fairly fall within the basic teaching hereis set forth.

Although the invention has been described with respect to specificembodiment for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodification and alternative constructions that may be occurred to oneskilled in the art which fairly fall within the basic teaching here isset forth.

The present invention is expected to be found practically use in thenovel photo-voltaic cells which as higher power generation capability(25 times and beyond) as compared with that of the conventional cells.The proposed invention can be used for fabricating wide solar panel forboth commercial and space applications.

1. A photovoltaic cell comprising: i. a first electrode wherein, thefirst electrode is substantially flat; ii. a layer of electronicmaterial disposed on the first electrode and electrically connected tothe first electrode, wherein the layer of electronic material is acontinuous layer; iii. a plurality of nanometer-scale wires extendinginto the layer of electronic material such that, space between theplurality of the nanometer-scale wires is substantially filled with thelayer of electronic material; and iv. a second electrode electricallyconnected to the plurality of the nanometer-scale wires, wherein, saidsecond electrode is not in electrical contact with the first electrodeand the layer of electronic material, wherein junctions providingbuilt-in electric field are formed at interfaces between thenanometer-scale wires and the layer of electronic material surroundingthe nanometer-scale wires, and wherein said junctions are ones selectedfrom a group consisting of p-n junctions and Schottky junctions.
 2. Thephotovoltaic cell of claim 1 wherein the layer of electronic materialcomprises p or n doped semiconductor, and the nanometer-scale wirescomprises n or p semiconductor doped opposite with respect to thenanometer-scale wires such that, the junctions formed are p-n junctions.3. A photovoltaic cell comprising: i. a first electrode wherein, thefirst electrode is substantially flat; ii. a layer of a first electronicmaterial disposed on the first electrode and electrically connected tothe first electrode, wherein the layer of the first electronic materialis a continuous layer; iii. a plurality of nanometer-scale tubes,extending into the layer of the first electronic material such that,space between the nanometer-scale tubes is substantially filled with thelayer of the first electronic material; and iv. a second electrodeelectrically connected to the nanometer-scale tubes, wherein, the secondelectrode is not in electrical contact with the first electrode and thelayer of the first electronic material, wherein junctions providingbuilt-in electric field are formed at interfaces between thenanometer-scale tubes and the layer of the first electronic materialsurrounding the nanometer-scale tubes, and wherein said junctions areones selected from a group consisting of p-n junctions and Schottkyjunctions.
 4. The photovoltaic cell of claim 3 wherein thenanometer-scale tubes are filled with a second material wherein, saidsecond material is in electrical contact with the first electrode andthe layer of the first electronic material; wherein, additionaljunctions providing built-in electric field are formed at interfacesbetween the nanometer-scale tubes and the second material, and whereinsaid additional junctions are ones selected from a group consisting ofp-n junctions and Schottky junctions.
 5. The photovoltaic cell of claim1 wherein the layer of electronic material includes a plurality ofdifferent layers such that the photovoltaic cell absorbs electromagneticradiation incident at different wavelengths.
 6. The photovoltaic cell ofclaim 5 wherein said different layers comprise of dissimilar material orsimilar material having different doping according to a prescribedcriterion such that the ensuing variation in absorption characteristicsresults in absorption of electromagnetic radiation over a widewavelength range.
 7. The photovoltaic cell of claim 3 where the layer ofthe first electronic material includes a plurality of different layerssuch that the photovoltaic cell absorbs electromagnetic radiation over awide wavelength range.
 8. The photovoltaic cell of claim 7 wherein, saiddifferent layers comprise of dissimilar material similar material havingdifferent doping according to a prescribed criterion such that theensuing variation in absorption characteristics results in absorption ofelectromagnetic radiation over a wide wavelength range.
 9. Thephotovoltaic cell of claim 1 further including: an electricallyconducting substrate in electrical contact with the second electrode;said substrate having one or more group of self-assembled dots orislands on at least one surface; wherein, the nanometer-scale wires aregrown on the one or more group of self-assembled dots or islands, suchthat the nanometer-scale wires are in electrical contact with the secondelectrode.
 10. The photovoltaic cell of claim 1 wherein: i. the firstelectrode is composed of a transparent conductive alloyed or non-alloyedmetal; ii. the nanometer-scale wires are composed of CdTe; iii. thelayer of electronic material electrically connected to the firstelectrode, is made of CdS; and iv. the second electrode is composed of ametal suitable to form ohmic contact with CdTe.
 11. The photovoltaiccell of claim 5, wherein the first electrode is composed of transparentconductive alloyed or non-alloyed metal; i. the nanometer-scale wiresare composed of GaN of n or p type; ii. each one of the plurality ofadditional layers electrically connected to the first electrodecomprises p or n type In_(1-X)Ga_(X)N, such that the doping polarity isof the opposite kind from one in the nanometer-scale wires; and iii. thesecond electrode is composed of a metal suitable to construct an ohmiccontact to GaN, wherein, in each one of the plurality of additionallayers, x is varied in a predetermined fashion such that, the layernearest to a surface receiving the incident electromagnetic radiationhas a high Ga content resulting in a high bandgap material layer, andthe layer farthest from said surface has low Ga content resulting in alow band gap material layer.
 12. The photovoltaic cell of claim 1,wherein the: i. plurality of nanometer-scale wires are physically andelectrically connected to a plurality of three-dimensional surfacestructures disposed on one surface of the second electrode, and whereinthe plurality of three-dimensional surface structures are electricallyconnected to the second electrode.
 13. The photovoltaic cell of claim 4,wherein the nanometer-scaled tubes comprise p or n doped semiconductor,the first electronic material and, the second material comprise n or psemiconductor doped opposite with respect to said nanometer-scaled tubessuch that, said junctions between the nanometer-scale tubes and thefirst electronic material and second material respectively, are p-njunctions.
 14. The photovoltaic cell of claim 1 wherein, the pluralityof nanometer-scale wires comprise of more than one material.
 15. Thephotovoltaic cell of claim 1 wherein, the plurality of nanometer-scalewires are constructed on a substrate which is in electrical contact withthe second electrode such that the plurality of nanometer-scale wiresare connected to the second electrode.
 16. The photovoltaic cell ofclaim 3 wherein, the plurality of nanometer-scale tubes are constructedon a substrate which is in electrical contact with the second electrodesuch that the plurality of nanometer-scale tubes are connected to thesecond electrode.
 17. A photovoltaic cell comprising: a first electrode,wherein the first electrode is substantially flat; a layer of a firstelectronic material disposed on the first electrode and electricallyconnected to the first electrode, wherein the layer of the firstelectronic material is a continuous layer; a second electrode, whereinthe second electrode is not in electrical contact with the layer of thefirst electronic material; a plurality of nanometer-scale wires or tubesphysically and electrically connected to a plurality ofthree-dimensional surface structures disposed on one surface of thesecond electrode such that, the plurality of nanometer-scale wires ortubes are electrically connected to the second electrode; wherein theplurality of the nanometer-scale wires or tubes extend into the layer ofelectronic material, such that the layer of electronic materialsubstantially fills the space between said nanometer-scale wires ortubes, such that junctions providing built-in electric field are formedat interfaces between the nanometer-scale wires or tubes and the layerof electronic material surrounding said nanometer-scale members, andwherein said junctions are p-n junctions or Schottky junctions.
 18. Thephotovoltaic cell of claim 17 wherein, said three-dimensional surfacestructures are electrodes in electrical contact with the secondelectrode.
 19. A photovoltaic cell comprising: a first electrode,wherein the first electrode is substantially flat; a plurality of layersof electronic material, wherein said plurality of layers of electronicmaterial is disposed on the first electrode and electrically connectedto the first electrode, wherein each of said plurality of layers ofelectronic is a continuous layer; a plurality of nanometer-scale wiresor tubes extending in the plurality of layers of electronic materialsuch that, the plurality of layers of electronic material substantiallyfill the space between the nanometer-scale wires or tubes; and a secondelectrode electrically connected to the plurality of nanometer-scalewires or tubes wherein, the second electrode is not in electricalcontact with the plurality of layers of electronic material; wherein,p-n junctions are formed at interfaces between the plurality ofnanometer-scale wires or tubes and the surrounding plurality of layersof electronic material.
 20. The photovoltaic cell of claim 19 wherein,different layers of said plurality of layers are arranged to absorbradiation of different wavelengths.